1. Field of the Invention
The present invention relates to a robot apparatus and a robot control method, which calculate a torsion of a joint to control an angle of the joint when the joint of a robot main body is driven by a rotary motor via a speed reducer.
2. Description of the Related Art
In general, a joint of a robot main body such as a robot arm is driven using a servo motor such as an AC servo motor or DC brushless motor, and a speed reducer is arranged on the output side of the servo motor so as to obtain a high output torque and is coupled to a structure member such as a link.
Conventionally, angle control of each joint of a robot arm is executed by detecting a rotation angle of a servo motor shaft using a rotary encoder (encoder) directly coupled to the servo motor shaft. However, since a speed reducer coupled to the servo motor suffers a torsion and cluttering in terms of its structure, a torsion is generated in a joint, and a joint angle cannot be precisely calculated based on the rotation angle of the servo motor shaft. For this reason, this results in a position error of the robot main body such as a robot arm distal end.
In recent years, in order to reduce a position error of the robot main body, a technique for detecting a position by an absolute encoder coupled to an output shaft of the speed reducer has been proposed (see Japanese Patent Application Laid-Open No. 2010-238174). Since a joint angle can be directly detected using the absolute encoder, a position error of the robot main body can also be reduced.
However, when the absolute encoder is coupled to the output shaft of the speed reducer like in Japanese Patent Application Laid-Open No. 2010-238174 above, a very high resolution is required for the absolute encoder.
More specifically, in a method in which the absolute encoder is mounted on a rotating shaft of a motor, since a resolution corresponding to a speed reduction ratio is relaxed in the speed reducer, a low-resolution encoder can be used. On the other hand, the absolute encoder arranged on the output shaft side of the speed reducer is required to have a high resolution obtained by multiplying the resolution of the encoder mounted on the motor by the speed reduction ratio. For example, when the speed reduction ratio of the speed reducer is designed to fall within a range of about 50 to 160, the absolute encoder arranged on the output shaft side of the speed reducer requires a resolution about 50 to 160 times that of the encoder mounted on the motor side.
In the absolute encoder, the number of detection elements such as light-emitting elements and light-receiving elements is increased with increasing resolution, resulting in a complicated structure, and increases in cost and size. For this reason, when the absolute encoder for joint angle detection is mounted on each joint of the robot main body, the cost and size of the robot apparatus are unwontedly increased.
Thus, the present invention provides a low-cost, compact robot apparatus, which can control each joint of the robot main body to a target joint angle with high precision.